Your Madagascar periwinkle will grow in full sun but looks best when protected from hot afternoon sun. Perhaps you just need to move it to a shaded area. The plant doesn't need a lot of water, so be careful not to overwater, even if it looks thirsty. If the strawberry jar is terra cotta, it should help keep the soil from staying soggy ater watering. (Terra cotta wicks the moisture out of the soil and helps evaporate it through the porous sides of the pot). Try moving the plant. If new leaves don't perk up, feed the plant with diluted liquid fertilizer.